OSI Model

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OSI Model
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OSI Model

• The OSI model is a conceptual model used to
  represent the means by which data is communicated
  between hosts and their network applications.

Host
Host
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OSI Layers

• The OSI Model is made up of seven basic layers.

7. APPLICATION
6. PRESENTATION
5. SESSION
4. TRANSPORT
3. NETWORK
2. DATA LINK
1. PHYSICAL
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PHYSICAL LAYER
7. APPLICATION
6. PRESENTATION
5. SESSION
4. TRANSPORT
3. NETWORK
2. DATA LINK
1. PHYSICAL

• Data arrives at the host through the Physical
  Layer and travels up through the successive
  layers as necessary for the arriving data to be
  processed.

Packets or Frames
Host
01101001001001010101
01101001001001010101
01101001001001010101
01101001001001010101
Bit Stream
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PHYSICAL LAYER
7. APPLICATION
6. PRESENTATION
5. SESSION
4. TRANSPORT
3. NETWORK
2. DATA LINK

• The Physical Layer of the OSI model consists of
  the media through which the data moves and the
  networking devices connected by that media.

1. PHYSICAL
Host1
Host2
PHYSICAL
PHYSICAL
01101001001001010101
01101001001001010101
01101001001001010101
01101001001001010101
REPEATERS
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PHYSICAL LAYER
7. APPLICATION
6. PRESENTATION
5. SESSION
4. TRANSPORT
3. NETWORK
2. DATA LINK
1. PHYSICAL
Host
Data packets arrive on the wire and are picked up
by the Network Interface Card (NIC) of the host
computer. The NIC is the interface between the
horizontal cabling and the host computer.
Network Interface Card (NIC)
01101001001001010101
01101001001001010101
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Data Link Layer
7. APPLICATION
6. PRESENTATION
5. SESSION
4. TRANSPORT
3. NETWORK
This layer provides reliable transit of data
across a physical link. The Data Link Layer is
concerned with physical addressing, network
topology, line discipline, error notification,
ordered delivery of frames, and flow control. The
IEEE has divided this layer into two sublayers
the MAC sublayer and the LLC sublayer. The Data
Link Layer is sometimes simply called the link
layer.
2. DATA LINK
1. PHYSICAL
Since the physical layer merely accepts and
transmits a stream of bits without any regard to
its meaning, it is up to the Data Link Layer to
create and recognize frame boundaries. This can
be accomplished by attaching special bit patterns
to the beginning and end of the frame. If there
is a chance that these bit patterns might occur
in the data, special care must be taken to avoid
confusion. The Data Link Layer provides error
control between adjacent nodes.
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Data Link Layer
7. APPLICATION
6. PRESENTATION
5. SESSION
4. TRANSPORT
3. NETWORK
2. DATA LINK
1. PHYSICAL
Host2
The MAC address of the NIC is recognized at this
layer.
DATA LINK
00-ED-09-F7-E7-23
Host1
PHYSICAL
PHYSICAL
01101001001001010101
01101001001001010101
01101001001001010101
01101001001001010101
REPEATERS
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Network Layer
7. APPLICATION
6. PRESENTATION
5. SESSION
4. TRANSPORT
3. NETWORK
2. DATA LINK
1. PHYSICAL

• Goal Move information across a network made of
  multiple segments
• Operates on basis of network addresses global in
• nature
• Routers and gateways operate at this level
• IP and IPX are examples
• Packet is reformatted for the receiving network

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Network Layer
7. APPLICATION
6. PRESENTATION
5. SESSION
4. TRANSPORT
3. NETWORK
2. DATA LINK
The Network Layer is concerned with controlling
the operation of the subnet. A key design issue
is determining how packets are routed from source
to destination. Routes could be based on static
tables that are "wired into" the network and
rarely changed. They could also be determined at
the start of each conversation, for example a
terminal session. Finally, they could be highly
dynamic, being determined anew for each packet,
to reflect the current network load. If too many
packets are present in the subnet at the same
time, they will get in each other's way, forming
bottlenecks. The control of such congestion also
belongs to the Network Layer.
1. PHYSICAL
When a packet has to travel from one network to
another to get to its destination, many problems
can arise. The addressing used by the second
network may be different from the first one. The
second one may not accept the packet at all
because it is too large. The protocols may
differ, and so on. It is up to the Network Layer
to overcome all these problems to allow
heterogeneous networks to be interconnected. In
broadcast networks, the routing problem is
simple, so the Network Layer is often thin or
even nonexistent.
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Network Layer
7. APPLICATION
6. PRESENTATION
5. SESSION
4. TRANSPORT
3. NETWORK
Host2
2. DATA LINK
1. PHYSICAL
NETWORK
209.129.42.146
DATA LINK
00-ED-09-F7-E7-23
Host1
PHYSICAL
PHYSICAL
01101001001001010101
01101001001001010101
01101001001001010101
01101001001001010101
REPEATERS
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Transport Layer

• Provides for reliable end-to-end delivery of data
  a logical connection
• Concerned with sequencing, error correction
• Breaks data into segments
• Sequence numbers, size, checksum in each packet
• Receipt at destination is verified
• Destination puts packets back into proper order
• TCP, SPX, UDP are examples
• Provides messaging service for the Session Layer
• May need to be very complex, or simple depending
  on other layers
• Multiplexing and de-multiplexing may be used
• Source and destination carry on conversation

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Transport Layer
7. APPLICATION
6. PRESENTATION
5. SESSION
The basic function of the Transport Layer, is to
accept data from the session layer, split it up
into smaller units if need be, pass these to the
network layer, and ensure that the pieces all
arrive correctly at the other end. Furthermore,
all this must be done efficiently, and in a way
that isolates the session layer from the
inevitable changes in the hardware technology.
Under normal conditions, the Transport Layer
creates a distinct network connection for each
transport connection required by the session
layer. If the transport connection requires a
high throughput, however, the Transport Layer
might create multiple network connections,
dividing the data among the network connections
to improve throughput.
4. TRANSPORT
3. NETWORK
2. DATA LINK
1. PHYSICAL
On the other hand, if creating or maintaining a
network connection is expensive, the Transport
Layer might multiplex several transport
connections onto the same network connection to
reduce the cost. In all cases, the Transport
Layer is required to make the multiplexing
transparent to the session layer. The Transport
Layer also determines what type of service to
provide to the session layer, and ultimately, the
users of the network. The most popular type of
transport connection is an error-free
point-to-point channel that delivers messages in
the order in which they were sent. However, other
possible kinds of transport include one where
there is no guarantee about the order of delivery
and another where messages are broadcast to
multiple destinations. The type of service is
determined when the connection is established.
The Transport Layer is a true
source-to-destination or end-to-end layer. In
other words, a program on the source machine
carries on a conversation with a similar program
on the destination machine using message headers
and control messages.
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Transport Layer
7. APPLICATION
6. PRESENTATION
5. SESSION
4. TRANSPORT
3. NETWORK
2. DATA LINK
1. PHYSICAL
Host2
TRANSPORT
Packet 1 - Packet 2 (Sequencing)
Host1
NETWORK
209.129.42.146
DATA LINK
DATA LINK
00-ED-09-F7-E7-23
01101001001001010101
REPEATERS
01101001001001010101
01101001001001010101
PHYSICAL
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Session Layer

• Establishes, synchronizes, maintains, and
  disconnects a communications link between two
  stations on a network
• Logging onto a network, connection made until you
  log out
• Transfer file between two machines
• Name to station address translation
• Data streams received from lower layers put into
  sessions

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Session Layer
7. APPLICATION
6. PRESENTATION
5. SESSION
4. TRANSPORT
3. NETWORK
2. DATA LINK
1. PHYSICAL
The Session Layer allows users on different
machines to establish sessions between them. A
session allows ordinary data transport, as does
the transport layer, but it also provides some
enhanced services useful in a some applications.
A session might be used to allow a user to log
into a remote system or to transfer a file
between two machines.
One of the services of the Session Layer is to
manage dialogue control. Sessions can allow
traffic to go in both directions at the same
time, or in only one direction at a time. If
traffic can only go one way at a time, the
Session Layer can help keep track of whose turn
it is. A related session service is token
management. For some protocols, it is essential
that both sides do not attempt the same operation
at the same time. To manage these activities, the
Session Layer provides tokens that can be
exchanged. Only the side holding the token may
perform the critical operation. Another session
service is synchronization. Consider the problems
that might occur when trying to do a two-hour
file transfer between two machines on a network
with a 1 hour mean time between crashes. After
each transfer was aborted, the whole transfer
would have to start over again, and would
probably fail again with the next network crash.
To eliminate this problem, the Session Layer
provides a way to insert checkpoints into the
data stream, so that after a crash, only the data
after the last checkpoint has to be repeated
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Session Layer
7. APPLICATION
6. PRESENTATION
5. SESSION
4. TRANSPORT
3. NETWORK
Host2
2. DATA LINK
1. PHYSICAL
1. PHYSICAL
\\MY_SERVER\RESOURCE
SESSION
TRANSPORT
Packet 1 - Packet 2 (Sequencing)
Host1
NETWORK
209.129.42.146
DATA LINK
DATA LINK
00-ED-09-F7-E7-23
01101001001001010101
REPEATERS
01101001001001010101
01101001001001010101
PHYSICAL
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Presentation Layer
7. APPLICATION
6. PRESENTATION
5. SESSION
4. TRANSPORT
3. NETWORK
The Presentation Layer performs certain functions
that are requested sufficiently often to warrant
finding a general solution for them, rather than
letting each user solve the problems. In
particular, unlike all the lower layers, which
are just interested in moving bits reliably from
here to there, the Presentation Layer is
concerned with the syntax and semantics of the
information transmitted.
2. DATA LINK
1. PHYSICAL
A typical example of a presentation service is
encoding data in a standard, agreed upon way.
Most user programs do not exchange random binary
bit strings. They exchange things such as names,
dates, amounts of money, and invoices. These
items are represented as character strings,
integers, floating point numbers, and data
structures composed of several simpler items.
Different computers have different codes for
representing character strings, integers and so
on. In order to make it possible for computers
with different representations to communicate,
the data structures to be exchanged can be
defined in an abstract way, along with a standard
encoding to be used "on the wire". The job of
managing these abstract data structures and
converting from the representation used inside
the computer to the network standard
representation is handled by the Presentation
Layer. The Presentation Layer is also concerned
with other aspects of information representation.
For example, data compression can be used here to
reduce the number of bits that have to be
transmitted and cryptography is frequently
required for privacy and authentication.
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Presentation Layer
7. APPLICATION
6. PRESENTATION
5. SESSION
Host2
4. TRANSPORT
3. NETWORK
3. NETWORK
PRESENTATION
The Quick qw62shwy8
2. DATA LINK
2. DATA LINK
1. PHYSICAL
1. PHYSICAL
\\MY_SERVER\RESOURCE
SESSION
TRANSPORT
Packet 1 - Packet 2 (Sequencing)
Host1
NETWORK
209.129.42.146
DATA LINK
DATA LINK
00-ED-09-F7-E7-23
01101001001001010101
REPEATERS
01101001001001010101
01101001001001010101
PHYSICAL
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Application Layer
7. APPLICATION
6. PRESENTATION
5. SESSION
4. TRANSPORT
3. NETWORK
2. DATA LINK
The Application Layer contains a variety of
protocols that are commonly needed. For example,
there are hundreds of incompatible terminal types
in the world. Consider the plight of a full
screen editor that is supposed to work over a
network with many different terminal types, each
with different screen layouts, escape sequences
for inserting and deleting text and for moving
the cursor.
1. PHYSICAL
One way to solve this problem is to define an
abstract network virtual terminal which editors
and other programs can be designed to deal with.
To handle each terminal type, a piece of software
must be written to map the functions of the
network virtual terminal onto the real terminal.
For example, when the editor moves the virtual
terminal's cursor to the upper left-hand corner
of the screen, this software must issue the
proper command sequence to the real terminal to
get its cursor there too. All the virtual
terminal software is in the Application Layer.
Another Application Layer function is file
transfer. Different file systems have different
file naming conventions, different ways of
representing text lines, and so on. Transferring
a file between two different systems requires
handling these and other incompatibilities. This
work, too, belongs to the Application Layer, as
do electronic mail, remote job entry, directory
lookup, and various other general-purpose and
special-purpose facilities.
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Encapsulation
Each layer adds information to the data on its
way to creating a frame or packet to be delivered
to the network. Once delivered to its
destination, the receiving host begins processing
the information added from the source to insure
the data has arrived intact and is routed
properly.
A layer will encapsulate the data with addressing
and control information.
As each layer performs specific functions, the
information it adds is used by the destination
host to properly route and process data and to
insure the integrity of the arriving data.
7. APPLICATION
6. PRESENTATION
5. SESSION
4. TRANSPORT
As the information is passed from layer to layer,
information added by the previous layer is used
to continue building the frame.
3. NETWORK
2. DATA LINK
1. PHYSICAL
Bit-Stream
01101001001001010101
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OSI Layer Relationships In Operating Systems
OSI
UNIX
NT
APPLICATION
Network File System
APPLICATION
PRESENTATION
SESSION
SNMP
FTP
SMTP
Telnet
TRANSPORT
TRANSPORT
TCP
INTERNET
IP
NETWORK
NETWORK INTERFACE
LAN Drivers
DATA LINK
Media Access Control
PHYSICAL
PHYSICAL
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